Today's trucks and sport utility vehicles (SUVs) require more cooling power than can be provided by electrically provided fans. In order to meet their cooling needs most automotive manufacturers utilize a viscous fan clutch to transmit power from the engine to the fan. A viscous fan clutch works by using a viscous fluid to create a mechanical link between two independent parts of the clutch, the clutch plate that is connected to a pulley powered by the engine and the fan drive body that is connected to the fan. To engage the clutch and bring the fan up to the speed of the pulley, fluid is released by a valve into an area between the clutch plate and the body. The fluid causes the two parts to drag together and the fan speed to increase. To disengage the fan clutch and reduce the fan speed, the valve is closed and the centrifugal force caused by the spinning of the fan drive causes the fluid to pump out of the area between the clutch plate and the body.
Typically either a bimetal coil or a magnetic actuator actuates the valve. There are advantages and disadvantages for both systems. The benefits of the bimetal coil include decreased cost, ease of implementation and decreased weight over the magnetic actuator. The magnet actuator has the benefit of computer control. This is not the case with the bimetal actuator. With the bimetal actuator the heat coming off of the radiator actuates the bimetal clutch. As such it can only be actuated based on one input, radiator temperature, while the magnetic actuator can be controlled by multiple inputs.
Therefore, there is a need to have a clutch with the controllability of the magnetically actuated clutch and also the size and cost benefits of the bimetal actuator.